As presently manufactured and used, phenol-resorcinol-formaldehyde adhesives constitute a compromise between the desirable high reactivity but undesirable high cost of resorcinol, and the desirable cost but undesirable lower reactivity of phenol.
Resorcinol-formaldehyde resins are made with a stoichiometric deficiency of formaldehyde, e.g. 0.7 mols per mol of resorcinol, and are quite storage-stable, being activated for use by adding formaldehyde to a ratio of 1 or more, whereupon gelation occurs cold.
Phenol-formaldehyde resins, by contrast, are made with a full complement of formaldehyde of 1.4 to 2.7 mols per mol phenol, have a commercially acceptable limitation of storage life, and are activated to set by heat in use operations typically characterized as hotpressing operations.
These disparate modes of storage and use of resorcinol-formaldehyde and phenol-resorcinol resins bring about several dilemmas when one seeks compromise intermediate storage life, cost, and setting speed in use in a phenol-resorcinol-formaldehyde combination. Generally, speaking, if phenol and resorcinol were both present in a formaldehyde reaction mixture, virtually no phenol would react before the resorcinol depleted the formaldehyde or gelled.
Therefore, it is usual commercial practice to first react phenol and formaldehyde to a polymer size resulting in a syrup of substantially increased viscosity, before charging the resorcinol late in the heated condensation period. The phenol-formaldehyde reaction is carried out with alkaline catalysis at a pH substantially greater than 8.5, but catalyst content is carefully limited because of deleterious effects of alkali on storage stability and adhesive durability. The formaldehyde ratio to phenol is held quite low because of severe effects on storage life when the resorcinol is added. The result is a poor utilization of the phenol present so that commercial phenol-resorcinol-formaldehye resins often have a strong characteristic odor of unreacted phenol.